Herbicides are in wide use in all types of agriculture at the present time. Agricultural research has established that the maximization of crop yields and economic return demands the use of appropriate herbicides to eliminate or at least reduce the competition of weeds for soil nutrients, water and sunlight. Many classes of herbicides are now in use. The great number of different crops which are grown economically, the wide variation in soil textures, and the extreme span of climatic conditions in which agriculture is pursued demand a wide range of herbicides of different characteristics. Agricultural chemical researchers continue to investigate possible new herbicides in the hope of finding compounds which have more advantageous properties than their predecessors.
Some pyrimidinone herbicides have been disclosed in the agricultural chemical art, such as the 6-alkyl-2,5-dihalo-3-phenyl-4-pyrimidinones of U.S. Pat. No. 3,823,135 Earlier workers have also found herbicides among the pyridazinones, for example, U.S. Pat. No. 3,644,355. A compound much like those of this invention, but lacking the substituent on the phenyl ring, has been disclosed in the chemical literature. Davies et al., A Novel Pyrimidine Synthesis, J. Chem. Soc. 347-51 (1945). This compound, however, has extremely low herbicidal activity.